Lithium-ion batteries are secondary (rechargeable) energy storage devices that are useful in laptop computers, power tools, and other portable devices that can use a source of relatively low potential electrical energy. Lithium-ion batteries are also being developed for automotive applications.
In a common embodiment, a lithium-ion battery cell comprises an anode (in the battery discharge mode the anode is the negative electrode) of lithium-intercalated graphite particles. Graphite is characterized by planes of strongly bonded carbon atoms, and with weaker bonding between the planes. Thus, lithium atoms are dispersed and diluted between the planes of carbon atoms in the graphite particles. These lithium-intercalated graphite particles may be deposited and bonded on a metal current collector substrate. The anode material is in contact with an electrolyte of a lithium salt, such as lithium hexafluorophosphate, dissolved in a non-aqueous solvent of mixed organic carbonates such as ethylene carbonate and dimethyl carbonate. The electrolyte, in turn, contacts a cathode of a composition, such as a transition metal oxide or phosphate, which accepts lithium ions transported from the anode through the electrolyte during discharge of the battery. When the lithium-ion cell is recharged, lithium ions are transported from the cathode through the electrolyte and intercalate into the graphite particles.
Such lithium intercalated anodes are commonly made by ball milling graphite particles in a low-boiling solvent (e.g., xylene) with a polymeric binder material (e.g., EPDM or PVDF) and a conductive additive (e.g., carbon black). The graphite-containing mixture is then solvent cast on a copper, nickel, or stainless steel current collector foil to form an anode assembly. During the operation of the lithium-ion battery cell, lithium is intercalated between the carbon planes of the graphite as lithium atoms. During discharge of the battery, lithium atoms in the anode are oxidized to lithium ions (Li+) which migrate from the graphite lattice, enter the electrolyte and flow into the cathode. The freed electrons from the oxidized lithium atoms enter the current collector and an external electrical load circuit, giving rise to current that can provide useful work (e.g., power an electric motor). The presence of an inactive binder and the substantially random alignment of the graphite particles on the anode current collector do not necessarily provide an efficient anode construction relative to the architecture we describe herein.
It is recognized that the atomic structure and organization of the anode in a lithium-ion cell plays a role in lithium-transport efficiency of the cell and its ability to experience repeated charges and discharges. There is a need for improved lithium-ion cell anode structures and a related and broader need to otherwise improve electrodes comprising metal ion intercalated carbon electrodes.